Evaluate at a particular set of parameters the 2-parameter logistic function.
logistic2_fn(x, theta)Numeric vector of the same length of x with the values of the
logistic function.
numeric vector at which the logistic function is to be evaluated.
numeric vector with the four parameters in the form
c(alpha, delta, eta, phi). alpha can only be equal to 0 or 1 while
delta can only be equal to 1 or -1.
The 2-parameter logistic function f(x; theta) is defined here as
g(x; theta) = 1 / (1 + exp(-eta * (x - phi)))
f(x; theta) = alpha + delta g(x; theta)
where theta = c(alpha, delta, eta, phi) and eta > 0. Only eta and phi
are free to vary (therefore the name) while vector c(alpha, delta) is
constrained to be either c(0, 1) (monotonically increasing curve) or
c(1, -1) (monotonically decreasing curve).
This function allows values other than {0, 1, -1} for alpha and delta
but will coerce them to their proper constraints.